BEIRUT -- A massive car bomb ripped through a crowded garage Thursday near a rebel-held border crossing between Syria and Turkey, killing at least 43 people in an area that has seen fierce fighting between rival rebel groups, an anti-government activist group said.

The attack came as President Bashar Assad's forces have seized the momentum of the country's 3-year-old civil war ahead of presidential elections scheduled for June 3.

The Britain-based Syrian Observatory for Human Rights said the blast killed 43 people and wounded more than 80. Injured Syrians taken to hospitals in Turkey and later died are among the 43 killed, said Rami Abdurrahman, the head of the Observatory. The group relies on a network of activists on the ground.

The Local Coordination Committees, another activist group, also reported the car bombing but said only that it killed and wounded "dozens of people."

Car bombings have become common in Syria as the influence of Islamic extremist groups has risen, dampening the support of the U.S. and its European allies for the opposition seeking to oust Assad. Opposition activists have blamed al-Qaida-linked fighters, who are engaged in deadly fighting between rival rebel factions in Syria, although no group claimed responsibility for Thursday's blast.

An amateur video posted online showed women, men and children at the scene of the blast near the Bab al-Salameh border crossing in the northern province of Aleppo. In another video posted online by activists, burned out cars are seen in the area near the crossing and the site of the attack, as people walk pass pools of blood, with clothes and other personal belongings scattered all around.

People cross the border at Bab al-Salameh on foot so the garage was filled with vehicles transporting people to or from the crossing.

"Oh God, may you punish them!" a man shouted as people used fire extinguishers to put out flames consuming two vehicles. The video appeared genuine and matched Associated Press reporting of the event.

In Turkey, a government official speaking on condition of anonymity because he wasn't authorized to release the information, said 94 wounded Syrians were brought across the border for treatment and that 14 of them had died.

Rebels fighting Assad captured the border crossing on the Syrian side in July 2012, opening a key transit point for people and supplies. But the area has seen an uptick in clashes and attacks between rebel groups fighting for control of the crossing in recent months.

The Syrian conflict started as largely peaceful protests against Assad's rule and turned into civil war after some opposition supporters took up arms to fight a brutal government crackdown on dissent. Activists say more than 150,000 people have been killed and millions have been driven from their homes, seeking shelter in neighbouring countries and in safer parts of Syria.

Thursday's attack came as U.S. Secretary of State John Kerry said the U.S. and other nations supporting the Syrian opposition agree that the planned election is a "farce" because it excludes Syrians who have been displaced by fighting. Kerry said he and his counterparts from 10 other nations agreed at a meeting in London to ramp up what can be done to assist the Syrian opposition. But he stopped short of promising any U.S. military aid.

Government forces also pressed on with their campaign to push rebels from the areas that could threaten Damascus, the seat of Assad's government, dropping bombs from a helicopter Thursday on the village of Tal Shihab, killing nine people, the Observatory said.

Meanwhile, the aid group Doctors Without Borders said five of its staff members kidnapped in northwestern Syria in January had been safely released. In a statement, the group said three staffers were released April 4 and the two others on Wednesday. It did not identify the staff members held, nor did it say if a ransom had been paid.

On Wednesday, at least 20 government soldiers were killed in a massive bombing after rebels detonated a tunnel packed with explosives under a military base in the Wadi Deif area of the northwestern province of Idlib. Reporting the attack Thursday, Abdurrahman did not explain how he got the figure, and Syrian authorities had no comment on the blast.

Another activist, Mohammed Kanaan, who is from the nearby town of Maaret al-Numan but is currently in Turkey, said the rebels had been digging the tunnel for about seven months. He said the tunnel was about 800 to 900 metres (yards) deep and filled with tons of locally made explosives.

An amateur video released by the Islamic Front, a coalition of seven rebel groups, showed debris flying meters up in the air as parts of the base exploded. The video corresponded with AP reporting about the attack. Kanaan also said the tunnel attack was carried out by the Suqour al-Sham, an Idlib-based group that is part of the Islamic Front.